1. Field of the Invention.
This invention relates to high strength, high cut-through resistant, polytetrafluoroethylene (hereinafter PTFE) insulation. This material can be used in electronics and other high technology fields.
2. Description of the Prior Art.
PTFE has become a very important insulation for the wire and cable industry. The use of this material has resulted in a new generation of insulated conductors with improved chemical, thermal, and electrical properties. Depending on the specific application, certain properties of the insulated wire are more desirable than others.
For example, PTFE which has been extruded and subsequently sintered by heating to a temperature above 345.degree. C. has highly desired electrical properties and finds numerous uses. One such use is in hook-up wire in the interconnection of electronic equipment. Military specifications require that hook-up wire passes a high voltage spark test, i.e. 2500 volts RMS for 6 mil thick insulation. PTFE hook-up wire satisfies these specifications. Also, PTFE has excellent dielectric properties including a low dielectric constant and low dissipation factor. PTFE insulation is used in applications where use temperature may be as high as 260.degree. C. or as low as near -273.degree. C. PTFE insulation is essentially chemically inert and therefore may be used in harsh chemical environments.
The disadvantages of this material, however, is that it has poor mechanical properties such as low cut-through resistance, low cold flow resistance, and low tensile strength. Typically, PTFE insulated wires have significantly lower cut-through resistance than such insulating materials as polyimide and polyester films and an extruded copolymer of ethylene and tetrafluoroethylene sold under the DuPont trademark, TEFZEL.RTM.. Cut-through resistance is the amount of force needed for a sharp edge to penetrate through the insulation wall and make contact with the conductor.
In 1970, a new form of PTFE was introduced (see U.S. Pat. No. 4,187,390). This high strength, expanded PTFE is an important insulation material used in the wire and cable industry. U.S. Pat. No. 3,953,566 teaches a method of making this material by expanding a paste-forming unsintered PTFE extrudate at high rates to produce strong porous materials. It has been found that these new materials in the unsintered form exhibit two crystalline melt points: one at approximately 342.degree. C. and another at approximately 384.degree. C.
Porous, expanded PTFE insulated wire and cable cannot be used as a hook-up wire because it does not have the desired electrical property of high voltage breakdown as required by the military specification. When multiple layers of this insulation are wrapped around a conductor, there can be poor adhesion between the multiple layers of insulation when it is sintered. This insulation also absorbs low surface tension liquids (e.g. less than 50 dynes/cm). As a result, the insulation cannot be used as the sole insulation in many applications.
Porous, expanded PTFE insulation, however, is highly desirable because of its excellent dielectric properties such as very low dielectric constant and very low dissipation factor.
The present invention provides an insulation that has the desired electrical property of high voltage breakdown as well as the desired mechanical property of increased cut-through resistance and the desired electrical properties, e.g. low dielectric constant and low dissipation factor.